The prior art uses optical cavities to filter electromagnetic radiation. Optical cavities are also used in conjunction with optical amplifiers to generate coherent radiation. More specifically, ring-type optical cavity filters can filter and generate radiation whose instantaneous frequency is changing in time. Changing the optical path length of the cavity causes the resonant modes of the cavity to shift in frequency at the same time rate as the desired frequency shift rate of the radiation to be filtered or generated. The optical path length of a cavity is changed with electro-optic devices such as rotating brewster plates as in the Model #699-05 ring cavity laser produced by Coherent Corp. or by electro-optic crystal devices such as the Series #620 phase/frequency modulators produced by Interactive Radiation, Inc. in Northvale, N.J. The optical path length of a cavity may also be changed by mechanically or piezoelectrically moving one of the mirrors which form the cavity. Electro-optic devices are currently not capable of causing path length changes of more than about 0.01% in a typical cavity. The mechanical and piezoelectric means are difficult to construct with sufficient precision for causing a cavity path length change corresponding to a frequency shift of more than approximately 100 resonant mode spacings for a one-meter long cavity. Further, shifting the frequency of the resonant modes of a cavity by changing the cavity length also changes the difference in frequency between two different modes.
In U.S. Ser. 370,434, filed Apr. 21, 1982 and completely incorporated herein by reference, a frequency shifted cavity was described whose stationary mode solutions actually have instantaneous frequencies which change in time. This cavity eliminated the need for changing the optical path length within the cavity and also eliminated the change in the difference in frequency between two different resonant modes. In the preferred cavity and laser described therein, the time rate at which the instantaneous frequency changed is determined by the frequency of an acoustic wave within an electromagnetic wave frequency shifter. The frequency shifted cavity for electromagnetic radiation is useful in the construction of optical amplifiers, electromagnetic wave cavity filters, scanning lasers and other electro-optic devices.
As the frequency of the electromagnetic radiation is filtered and/or scanned, it eventually moves outside the Bragg condition of the frequency shifter acousto-optic cell in the cavity. It would be highly desirable to have an apparatus and a method for extending the frequency range over which the Bragg condition is maintained, thus extending the filter range of the cavity and scan range of the laser. It would also be desirable to have a filter and laser whose frequency shift increases non-linearly with time.